This invention relates to an effective technology which is applied to a laser wavelength measuring device to be used in correcting a displaced laser wavelength in a laser device.
A laser beam has some features of a coherent high wavelength purity and a high output or the like and has been applied as an effective light source capable of giving a high beam radiation. In recent years, there has been developed a light source device utilizing such a laser beam. A typical device is a narrow band excimer laser which is studied as a light source for a lithography used in an ultra-reduced projecting and exposure process applied in a semiconductor device manufacturing step.
In this type of laser device applied to this kind of application, it was necessary to always monitor a displacement of wavelength of the projected laser beam in order to stabilize the laser oscillation wavelength so as to perform a feed-back control of the laser oscillation device.
As a configuration of the measuring device in such a monitor system, it was a usual process to change a radiated laser beam into a dispersion light formed into some spectrum with a spectrum analyzer such as etalon or the like and then to receive the light with a light receiving element such as a photo-detector array or the like. In this case, the aforesaid etalon is a wavelength selecting element in which multiple reflection and interference phenomena produced between a pair of reflection films are applied, and it has a function to radiate an incident light onto the light receiving element in an interference stripe. The aforesaid photo-detector array is defined as one in which light receiving elements such as photo-diodes or the like are arranged in a linear direction and a distribution of intensity of light is detected by monitoring an electric current generated under an optical excited state in each of the light receiving elements.
The aforesaid etalon shows a substantial variation in its characteristic in response to a variation of temperature and atmospheric pressure in an applied environment or the like. In the prior art, it was a usual practice to store the etalon applied to such an application described above in an air-sealed container of which temperature and air pressure are kept constant. In addition, a light having a reference wavelength (a reference light) was simultaneously received together with a laser beam from the laser device acting as a measured beam in response to the aforesaid variation in characteristic so as to compare both wavelengths and to detect a displacement of the wavelengths and then the feed-back control was carried out in reference to this data.
However, as described above, in case where the measured beam and the reference beam are received by the same optical path, the aforesaid photo-detector array measurement produces a relatively easy detection of a position of the maximum intensity, its signal processing for the measured beam and the reference beam not only became quite complicated, but also its processing speed was reduced and a controlling characteristic was apt to be deteriorated. That is, in case that the measured beam and the reference beam were radiated simultaneously, the measured beams and the reference beam should be discriminated in reference to the signal from the photo-detector array and the calculation process of many steps was required in order to compare the difference at both central wavelength positions.